You trust the security of your CPU vendor in all cases. SGX doesn't change that. If Intel wanted to, they could release a microcode update that detects a particular code sequence running and then patches it on the fly to create a back door. You'd never even know.
"SGX as a concept for tenant-provider isolation requires strong local attacker security, which is something off the shelf x86 has never had"
Off the shelf CPUs have never had anything like SGX, period. All other attempts like games consoles rely heavily on establishing a single vendor ecosystem in which all code is signed and the hardware cannot be modified at all. Even then it often took many rounds of break/fix to keep it secure and the vendors often failed (e.g. PS3).
So you're incorrect that Intel is worse than other vendors here. When considering the problem SGX is designed to solve:
- AMD's equivalents have repeatedly suffered class breaks that required replacing the physical CPU almost immediately, due to simple memory management bugs in firmware. SGX has never had anything even close to this.
- ARM never even tried.
SGX was designed to be re-sealable, as all security systems must, and that more or less has worked. It's been repeatedly patched in the field, despite coming out before micro-architectural side channel/Spectre attacks were even known about at all. That makes it the best effort yet, by far. I haven't worked with it for a year or so but by the time I stopped the state of the art attacks from the research community were filled with severe caveats (often not really admitted to in the papers, sigh), were often unreliable and were getting patched with microcode updates quite quickly. The other vendors weren't even in the race at all.
"there was absolutely no excuse for L1TF and some of the others, and those really showed us just how security-oblivious Intel's design teams are"
No excuse? And yet all CPU vendors were subject to speculation attacks of various kinds. I lost track of how many specex papers I read that said "demonstrating this attack on AMD is left for future work" i.e. they couldn't be bothered trying to attack second-tier vendors and often ARM wasn't even mentioned.
I've seen some some security researchers who unfortunately seemed to believe that absence of evidence = evidence of absence and argued what you're arguing above: that Intel was uniquely bad at this stuff. When studied carefully these claims don't hold water.
Frankly I think the self-proclaimed security community is shooting us all in the foot here. What Intel is learning from this stuff is that the security world:
a. Lacks imagination. The tech is general purpose but instead of coming up with interesting use cases (of which there are many), too many people just say "but it could be used for DRM so it must die".
b. Demands perfection from day one, including against attack classes that don't exist yet. This is unreasonable and no real world security technology meets this standard, but if even trying generates a constant stream of aggressive PR hits by researchers who are often over-egging what their attacks can do, then why even bother? Especially if your competitors aren't trying, this industry attitude can create a perverse incentive to not even attempt to improve security.
"the x86 world would probably do well to listen to Microsoft, since their Xbox division managed to coax AMD into actually putting out secure silicon"
SGX is hard because it's trying to preserve the open nature of the platform. Given how badly AMD fared with SEV, it's clear that they are not actually better at this. Securing a games console i.e. a totally closed world is a different problem with different strategies.